1. Field of the Invention
This invention relates to a discharge lamp and more particularly to a discharge lamp having a heater tightly attached to the outer surface of the bulb.
2. Description of the Related Art
Recently cold cathode fluorescent lamps and hot cathode fluorescent lamps are used for the back light for liquid crystal televisions and liquid crystal displays. The liquid crystal televisions and liquid crystal displays are sometimes used in environments with harsh temperature conditions. For example, fluorescent lamps used as the back light of the automobile meters are used in a wide temperature range of from about +40.degree. C. to -30.degree. C. Even in such a severe temperature condition, the lamps are required to have sufficient startability and rise-up characteristics.
A well-known method of improving the rise-up characteristics of lamps in a cold climate is to attach a heater to the outer surface of the bulb. In lamps with such a heater, the heater is supplied with electricity to generate heat to make the bulb warm, thereby facilitating the evaporation of mercury contained in the bulb. Thus, the vapor pressure of the mercury can be raised rapidly. As a result, the lamp can be started and the rise-up of the luminous flux is quickened, thereby reducing the time required for the lamp to reach the stable lighting condition.
When a heater is used, the temperature distribution of the lamp should be uniform at least in the axial direction of the bulb. Therefore, the generally adopted construction of discharge lamps is such that a heater in a band shape is placed in the axial direction of the bulb.
A heater of band shape must be secured in close contact with the outer surface of the bulb. For this reason, the heater is required to be in a thin plate and flexible, that is, deformable to cover the outer surface of the bulb, like a mesh heater. A possible fixing method of such a flexible heater to the outer surface of the bulb is to glue the heater to the bulb outer surface with an adhesive. In this case, however, there is a possibility of the heater coming off due to the deterioration with time of the adhesive. Further, it takes time for the adhesive to be dried, resulting in a longer manufacturing time of the lamp. Therefore, proposed is a construction that a heater is fixed to the outer surface of the bulb by means of a light-transmitting tube, particularly, by a heat-shrinkable tube, fitted over the full length of the bulb.
However, flexible heaters in a band shape are low in mechanical strength and are likely to be deformed or subject to breaking when they are handled. When a flexible heater is fixed to the bulb by a heat-shrinkable tube, since the tube does not uniformly shrink in the axial direction, the heater is likely to have stresses induced inside and have wrinkles. If deformation such as wrinkles occurs in the heater, the bulb cannot be heated efficiently and there is a high possibility that discontinuity occurs in the heater.